The present invention relates to a method of fabricating a semiconductor device. More particularly, it is concerned with a method of fabricating a semiconductor device capable of selectively etching and thereby removing a film formed from a silicide of a metal selected from various metals.
As well known, with improvement in integration density of a semiconductor integrated circuit, silicides of various metals, including tungsten, molybdenum, platinum, palladium and tantalum, have come to be used. However, the etching rates of these silicides are not so greatly different from the etching rates of singlecrystalline silicon, polycrystalline silicon and silicon dioxide, so it has been difficult to selectively remove a desired portion of a silicide film and accurately form electrodes and interconnection in a semiconductor device.
For example, where a desired portion of a silicide film formed on a single-crystalline silicon substrate is to be selectively removed by etching, it is necessary to perform an overetching in order to effect etching completely. In this case, however, since single-crystalline silicon and silicide are not so different in etching rate as mentioned above, even the single-crystalline silicon substrate of the portion exposed after removal of the silicide film is etched to a remarkable extent.
Heretofore, as the method of selectively removing a silicide film, there has been used (1) a method of removing an unnecessary silicide film portion according to a lift-off process or (2) a method in which the etching rate of a silicide film under predetermined conditions is measured in advance and a desired portion of silicide is removed with reference to the measured value.
However, in the above method (1), it is likely that removed silicide pieces which have entered a resist removing solution during lift off will adhere as a foreign matter onto the surface of a substrate under treatment. Such an adhesion of a foreign matter may result in the semiconductor device being no longer able to fulfill its function. In the above lift-off method (1), moreover, it is necessary that the substrate temperature at the time of forming the silicide film be held not higher than 100.degree. C. in order to prevent the generation of contaminant from the molten resist. At such a low temperature, however, even if there is made an attempt to form a silicide film by a chemical vapor deposition (CVD) or sputtering, it is impossible to form a uniform silicide film throughout the surface because of an extremely low coating ratio. Thus, it is very difficult to practically adopt the above method (1).
In the above method (2), in order to compensate for the non-uniformity of the silicide film thickness, there is performed an overetching for the purpose of completely removing a desired portion of the silicide film. In this case, there arises the problem that the silicon layer formed under the silicide film is also etched and removed. In the above method (2), moreover, there is fear of occurrence of the problem that the etching rate under the above etching condition differs markedly according to silicide film composition ratios.
As a reform measure for the method (2), the following method (3) is proposed in M. Y. Tsai et al, "One-Micron Polycide (WSi.sub.2 on Poly-Si) MOSFET Technology" in J. Electrochem. Soc., vol. 128, 2207 (1981).
In the method (3), using a plasma etching process, the reaction gas flow rate is set to CF.sub.4 :O.sub.2 =1:3 (volume ratio) to thereby improve the etching selection ratio of silicide to silicon. It is described in the above literature that an etching rate selection ratio of WSi.sub.2 to an n-type high concentration polycrystalline Si of about 4.5 is sometimes obtained. According to this method, even if a WSi.sub.2 film is formed on an n-type high density concentration polycrystalline silicon film and the former is overetched, it is possible to remove a desired portion of WSi.sub.2 selectively while somewhat suppressing etching of the underlying n-type high concentration polycrystalline silicon. However, the method (3) involves the following problems. The first problem is that optimum conditions for plasma etching differ according to silicide composition ratios. At present, for example the composition ratio of WSi.sub.2 film varies in the range of W:Si=1:2-2.3 according to silicide forming methods, so it is necessary to change etching conditions according to a change of the composition ratio.
The second problem is that if the substance present under the silicide film differs, for example like a p-type high concentration polycrystalline silicon, a non-doped polycrystalline silicon or a single-crystalline silicon, the foregoing selection ratio between the silicide film and the underlying substance differs depending on the kind of the underlying substance, thus making it difficult to etch only a desired portion of the silicide film selectively with a high accuracy.
The third problem is that a change of the reaction gas mixing ratio causes a great change of the etching selection ratio between the silicide film and the polycrystalline silicon, so it is difficult to etch a desired portion of the silicide film selectively while preventing damage of the undercoat.
Thus, various problems have been involved in the conventional methods for selectively removing the silicide film.